A4 indicates that the austenitic stainless steel alloy grade (4) has been cold worked (A), while the designation after A4 is indicative of the minimum tensile strength that is to be met by the material or in this case fasteners. Hence, A4-70 Hex Bolt would mean that these are cold worked bolts produced from austenitic stainless steel and the alloy has a minimum tensile strength requirement of about 700 Mpa.

A4-70 Hex Bolt

A4-70 Hex Bolt

The A4 50 Bolts or as referred to as per the BS EN ISO standard by ISO 3506 is a Marine grade stainless steel. This marine grade stainless steel has increased contents of both chromium and molybdenum which increases the corrosion resistance properties of the alloy. Molybdenum in this alloy is what improves its resistance to pitting and crevice corrosion as well. These features are observed especially when the alloy has been exposed to chlorides and sulphur containing environments.


On the other hand, Chromium when added to the alloy of A4-80 Stainless Steel Stud bolts, increases the alloy’s resistance to oxidation. Due to a significant content of chromium, the A4 stainless steel threaded rod forms a self-repairing passive Oxide layer, thus preventing oxidation. This acid resistant Stainless Steel A4-70 Nuts is well suited for use in harsh chemical environments and outdoor environments. While the properties of A4-80 stainless steel screws are similar to A4 in terms of its corrosion resistance properties, the only difference is that A4-80 has a marked increase in its tensile strength properties, which is handy for mechanically demanding applications.

Standard Specification of A4-80 Stainless Steel Stud bolt

Sizing 3/6 inch upto 2 inch | M3 – M56
Specifications ASTM A193, A194, F594, F593, F1554
Size of Machine Screw M1.6 - M12
Lengths From 3 upto 200 Millimetre

Chemical Structure of A4-70 Hex Bolt

Grades   C Si Mn P S Cr Mo Ni Cu N
A4 Min - - - - - 16.0 2.0 10.5 - -
ISO Max 0.08 1.00 2.00 0.045 0.03 18.5 3.0 14.0 1.0 -
A4-70 Min - - - - - 16.0 2.0 11.5 - 0.12
ISO² Max 0.03 1.00 2.00 0.045 0.03 18.5 3.0 14.0 1.0 0.22

A4 50 Bolts Mechanical Properties Tensile Strength

Product Yield Strength
0.2% Proof
(MPa) min
Tensile Strength (MPa) min Elong
(% in 50mm) min
Brinell (HB) max Hardness
Rockwell B (HR B) max
A4 450 700 40 Not required Not required